Northern Rock has sold off some of its most valuable mortgage assets to repay just over £2bn of its £26bn Bank of England loan.

The Newcastle-based bank announced this morning that it has sold its portfolio of lifetime home equity release mortgages to JP Morgan for a small premium on their £2.2bn book value.

The move is a coup for ministers who have come under sustained attack for their handling of the Northern Rock affair. It also comes ahead of a shareholders meeting next week at which a group of investors led by two activist hedge funds are expected to attack the government and the board for their failure to protect shareholders' interests.

Analysts said they were concerned the sale meant the stricken bank had lost control of some its best assets. Equity release mortgages are typically sold to older people who want to benefit from rising house prices without having to sell up, and are therefore of higher quality than other Northern Rock mortgages - such as those guaranteed on 125% of the value of a property.

The bank's chief executive, Andy Kuipers, said that while it was a "relatively small transaction" it was a "positive development" in the company's ongoing strategic review.

"It illustrates the quality of our assets, which has enabled us to achieve a sale at a premium despite continuing difficult financial markets, and will allow the company to reduce its debt to the Bank of England," he said.

"If the money from this sale is used to reduce further borrowing from the government, then it is probably desirable," he said.

But the Unite union claimed that the move could symbolise a "piecemeal sell off" of the bank's assets.

"Unite will continue to oppose any asset stripping of Northern Rock and fight for the company to remain as a single financial entity based in the north-east," said Unite's deputy general secretary, Graham Goddard.

The government is still trying to find a buyer for the bank.

These efforts may not be helped by the discovery of a £100m black hole in its employee pension fund.

The scheme's trustees warned members today that they are concerned that the Newcastle-based bank may be unable to continue paying into the fund.

"The significant deterioration in the financial position of Northern Rock, which necessarily implies a more pessimistic view being taken of the company's ability to support the scheme in the future," the trustees explained in a letter.

The news means more worry for Northern Rock employees, who have already suffered months of uncertainty since the bank's near-collapse in September. If the bank were to be sold, the cost of winding up the fund could rise to £200m.

The trustees also revealed that they had transferred almost all of the scheme's assets into government gilts, and out of shares, following the drop in stock markets in recent months.

"As a result, the final salary section's assets are very well protected from the possibility of falling stock market values," they said.

Cable said he is also concerned by a report on the BBC website yesterday that the government is planning to convert between £12bn and £15bn of the taxpayer Rock loans into triple-A rated bonds for sale to international investors.

"While this may be one way of providing security it could mean that the government will not be repaid for many years," Cable said.

"This raises big questions about whether the government will seek to hide this debt by taking it off its balance sheet."